Chap. XIJ THE VASCULAR SYSTEM 201 



communications with each other, and the anastomoses of veins 

 are always more numerous than those of arteries. 



The systemic veins. — The systemic veins are naturally divided 

 into two groups : — 



1. Those from which the blood is carried to the heart by the 

 supsrior vena cava, viz. the veins of the head, neck, upper extremi- 

 ties, and the walls of the thorax. 



2. Those from which the blood is carried to the heart by the 

 inferior vena cava, viz. the veins of the lower limbs, the lower 

 part of the trunk, and the abdominal viscera. 



Coronary veins. — In this group we may include the veins of 

 the heart, which, however, pass directly into the right auricle 

 without entering the superior vena cava. 



Veins of the head and neck. — The blood returning from the 

 head and neck flows on each side into two principal veins, the 

 external and internal jugular. 



External jugular veins. — The right and left external jugular 

 veins are formed in the substance of the parotid glands by the 

 union of two of the veins of the face. This union takes place on a 

 level with the angle of the lower jaw, and each vein descends almost 

 vertically in the neck to its termination in the subclavian. These 

 two veins receive the blood from the face and the exterior of the 

 cranium. 



Internal jugular veins. — These veins begin at the base of the 

 skull and descend on either side of the neck, first" with the external 

 carotid, then with the common carotid, and join at a right angle 

 with the subclavian to form the innominate (brachio-cephalic) 

 vein. They receive the blood from the veins and sinuses of the 

 cranial cavity. (See Fig. 131.) 



Sinuses. — The blood from the interior of the skull is returned 

 to the large veins by venous channels that are called sinuses. 

 They are formed by a separation of the layers of the dura mater, 

 the fibrous membrane which covers the brain. Their outer wall 

 consists of the dura mater, and their inner lining of endothelium is 

 continuous with the lining membrane of the vessels that com- 

 municate with them. (See Fig. 184.) 



Veins of the upper extremities. — The blood from the upper 

 limbs is returned by a deep and a superficial set of veins. The deep 

 veins are the venae comites of the forearm and arm and are called 



